WDM channel monitoring system and method

ABSTRACT

The system and method of the present invention is operative to monitor the performance of an optical communications channel. An optical splitter splits a wavelength division multiplexed (WDM) optical communications signal into a low power WDM signal onto a back-up path, where a tunable filter sweeps the optical communications channels, while a monitoring circuit monitors the optical communications channels for performance. Optical power can be stored and subsequently displayed, providing an optical spectrum analysis of the signal.

FIELD OF THE INVENTION

This invention relates to the field of optical communication systems,and more particularly, this invention relates to an opticalcommunication system using wavelength division multiplexed (WDM) opticalcommunication signals and having back-up receiver capability.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing (WDM) is commonly used in opticalcommunication systems for increasing the bandwidth of a fiber optictelecommunications link, without increasing the speed of associatedelectronics. In many prior art optical communication techniques, thebandwidth of a single channel (or wavelength fiber optictelecommunication link) has been limited primarily by the high-speedelectronics required at the transmitter and receiver. By usingwavelength division multiplexing at a telecommunications systemreceiver, the optical channels that receive the optical communicationsignals are separated, or demultiplexed, and sent to individualreceivers, which vary in their rate of data receipt. One example of areceiver is a 2.488 Gb/S receiver.

The number of individual receivers used in the optical communicationssystem can vary. These communication receivers connect into a back planeof existing telecommunications equipment. For example, atelecommunications rack could include one or more receivers, such as 8or 16 receivers, each mounted on a board within the telecommunicationsrack. When optical components fail, it is necessary to determine thechannel that is being used by the failed optical component or particularreceiver.

In the past, telecommunication links have rerouted signals on theelectrical switching level when any optical components failed, thusloading another path onto the network. It would be more advantageous tore-route an optical communication signal on a particular wavelengthchannel at the receiver terminal, in the case of a receiver failure orother optical component failure, and not consume network bandwidth as inprior art techniques. This would allow receiver maintenance at any timewithout increasing downtime or network re-routing.

It would also be desirable to monitor a channel and allow continuoussweeping of the optical communications channels. For example, if achannel showed any signs of weakening or failure, it would beadvantageous to identify the source of the problem so that correctivemeasures could be sought. Thus, there is a need for greater channelmonitoring capability. Although there are some channel monitoringdevices that use single mode fiber, such as one commercially availablesystem manufactured under the trade designation “Spectra SPAN,” it hasno capability as a back-up signal receiver.

SUMMARY OF THE INVENTION

The present invention is advantageous and allows the re-routing ofoptical communication signals at the receiver terminal, in case ofreceiver failure or other optical component failure. The system alsodoes not consume network bandwidth as in past practices, where signalshave been re-routed on the electrical switching level when opticalcomponents failed. Thus, in the present invention, another path is notloaded onto the network and bandwidth is not consumed. The presentinvention also allows receiver maintenance at any time, without downtime or network re-routing.

The present invention can also function as a channel monitor, allowingcontinuous sweeping of optical communication channels for quality andperformance. When a channel shows signs of weakening or failure,identification of the source of the problem can be triggered, andcorrective measures sought. If any one of the dedicatedtelecommunications system receivers fail on any given wavelength, theback-up receiver system of the present invention can be tuned to thatparticular wavelength and take over the link, while repairs are beingconducted.

The present invention can also be used as a tracking filter for systemsthat use a tunable laser for laser transmitters that fail. The receivercan track to a new wavelength location where a tunable transmitter hasbeen positioned to account for a failing, or a failed laser transmitter.The present invention can also be used as a tunable receiver forsystems/locations requiring tunability, such as add/drop nodes on afiber.

In accordance with the present invention, the system monitors theperformance of an optical communications channel and includes an opticalsplitter positioned along an optical communications path for receiving awavelength division multiplexed (WDM) optical communications signal onthe optical communications path. This signal is split into a low powerWDM signal onto a back-up path where a tunable filter receives the lowpower WDM optical signal and sweeps the optical communications channels.A monitoring circuit is operatively connected to the tunable filter andmonitors the optical communications channels for performance. Thetunable filter is swept and the optical power is stored and subsequentlydisplayed, providing an optical spectrum analysis of the signal. Theoptical amplifier can receive the low power WDM signal and amplify sameafter splitting from the optical communications signal.

In another aspect of the present invention, the amplifier includes aninjection laser diode and a current source control loop circuitconnected to the injection laser diode that establishes a fixed currentthrough the injection laser diode. A voltage switcher circuit isconnected to the injection laser diode and current source control loopcircuit. The tunable filter can comprise a Fabry Perot filter. Thecontroller can be operatively connected to the tunable filter in acontroller feedback path for controlling the selection of desiredwavelengths corresponding to the optical communications channels.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1 is a block diagram of a wavelength division multiplexed opticalcommunications system having a plurality of fixed receivers and atunable optical receiver of the present invention positioned on theback-up path, which are connected to the back plane of existingtelecommunications equipment.

FIG. 2 is another block diagram showing add/drop nodes, where a tunablereceiver, processing equipment, and tunable transmitter are used.

FIG. 3 is another block diagram showing an example of a wavelengthdivision multiplexed optical communications system having the opticallyamplified back-up receiver of the present invention.

FIG. 4 is another block diagram similar to FIG. 3, but showing ingreater detail the optically amplified back-up receiver of the presentinvention.

FIG. 5 is an enlarged block diagram of the tunable filter of the presentinvention having optical channel monitoring capability with a spectrumanalyzer.

FIG. 6 is a block diagram of a low power laser diode driver used as partof the amplifier section of the optically amplified back-up receiver ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

The present invention is advantageous and allows the re-routing ofoptical communication signals at the receiver terminal, in case ofreceiver failure or other optical component failure. The system alsodoes not consume network bandwidth as in past practices, where signalshave been re-routed on the electrical switching level when opticalcomponents failed. Thus, in the present invention, another path is notloaded onto the network and bandwidth is not consumed. The presentinvention also allows receiver maintenance at any time, without downtime or network re-routing.

The present invention can also function as a channel monitor, allowingcontinuous sweeping of optical communication channels for quality andperformance. When a channel shows signs of weakening or failure,identification of the source of the problem can be triggered, andcorrective measures sought. If any one of the dedicatedtelecommunications system receivers fail on any given wavelength, theback-up receiver system of the present invention can be tuned to thatparticular wavelength and take over the link, while repairs are beingconducted.

The present invention can also be used as a tracking filter for systemsthat use a tunable laser for laser transmitters that fail. The receivercan track to a new wavelength location where a tunable transmitter hasbeen positioned to account for a failing, or a failed laser transmitter.The present invention can also be used as a tunable receiver forsystems/locations requiring tunability, such as add/drop nodes on afiber.

FIG. 1 illustrates an optical communication system at 10, where awavelength division multiplexed (WDM) optical communications signal 12is transmitted along optical communications path 13. The opticalcommunications signal 12 passes through an optical splitter 14,positioned along the path 13, which splits off about 5%, as anon-limiting example, of the light power comprising the opticalcommunications signal and as a full spectrum of that signal into aback-up path 15. About 95% of the wavelength division multiplexedoptical communication signal passing along the optical communicationspath continues into a WDM circuit 16, which includes a demultiplexer fordividing the WDM optical communications signal into separate opticalcommunication signals of different wavelengths λ1 through λn, and intorespective fixed (dedicated) receivers 18, such as the illustrated fixedreceiver 1, fixed receiver 2, . . . fixed receiver n. The fixedreceivers 18 connect into the back plane 20 of existingtelecommunications equipment, as known to those skilled in the art.

The optical signal 22 is formed from the split-off portion of the lightand comprises about 5% of the original power of the WDM opticalcommunications signal 12. It is a low power WDM optical signal havingthe full spectrum of the light from the original WDM opticalcommunications signal 12. The tunable, optically amplified back-upreceiver 24 of the present invention receives the optical signal 22 andselects an optical signal of desired wavelength for the appropriatechannel to be backed-up (λ1 through λn), and converts this opticalsignal of desired wavelength into an electrical communications signal tobe fed into the back plane 20. The receiver 24 of the present inventionoperates as a back-up receiver in case one of the fixed receivers 18 isinoperable, or the optical components that carry an optical signal of aparticular wavelength are inoperable.

FIG. 2 illustrates how the tunable, optically amplified back-up receiver24 of the present invention can be used in a system or locationrequiring tunability, such as for respective add/drop nodes 26,28 on atelecommunications fiber. The tunable receiver 24 is operativelyconnected to telecommunications processing equipment 32 and a tunabletransmitter 34. The optical signal of selected wavelength can be droppedand received in the tunable receiver 24. It is converted by the receiver24 into the appropriate electrical communications signal, which is thenprocessed by appropriate signal processing circuitry, amplifiercircuitry, regeneration circuitry and other circuitry known to thoseskilled in the art. Once processed, the electrical communications signalis passed to the tunable transmitter 34, which converts the electricalcommunications signal that had been processed into an optical signal. Itis then added to the main optical communications signal 12 passing alongthe main optical communications path 13.

FIG. 3 illustrates a high level block diagram of the opticalcommunications system 10 where optical communications signals 35 ofabout 1550 nanometers are wavelength division multiplexed 36 intooptical communications signal 12 at about 155 Mb/S to about 4 Gb/S in ahigh bandwidth data distribution system 37, including appropriatein-line, erbium doped fiber amplifiers 38 acting as optical repeaters.The amplified optical communications signal 12 is passed to thededicated optical receivers 18 along the main optical communicationspath 13.

The optical splitter 14 forms a node that allows the full spectrum ofthe wavelength division multiplexed optical communications signal to besplit off (about 5% of its power) and passed into back-up path 15 as anoptical signal 22 and to the tunable mini/low power optically amplifiedback-up receiver 24 of the present invention.

The tunable optically amplified back-up receiver 24 of the presentinvention includes an erbium doped fiber amplifier 44 (EDFA) acting as apreamplifier. This permits amplification of the low power optical signalbefore passing into the tunable bandpass filter 46, which selects one ofthe desired wavelengths, λ1 through λn. A photodetector, which in thepresent embodiment is a PIN diode 48, but also can be an Avalanche PhotoDiode (APD), converts the amplified and optical signal of desiredwavelength into an electrical communications signal and passes thatelectrical communications signal into a low-noise electrical amplifier50 and into the clock and data recovery circuit 52.

FIG. 4 illustrates greater details of the tunable optically amplifiedback-up receiver 24 of the present invention, and illustrating threemain sections as an amplifier section 54, having the erbium-doped fiberamplifier (EDFA) 44 as shown in FIG. 3, a tunable filter section 56, andthe receiver section 58 operable as a detector used at differentwavelengths. The detector electronics is selected to support typicaldata rates, including 2.5 and 10.0 Gb/S.

Although the ranges of data and number of used channels are set forth asnon-limiting examples, it should be understood that the presentinvention is advantageously used with different wavelengths anddifferent number of channels. As illustrated, the WDM opticalcommunications signal, such as 2.5 Gb/S WDM signal input, passes into a1550/980 WDM input circuit 58 a that is operable with a Fiber BraggGrating Stabilized Pump Laser Diode circuit 59 and a low power laserdiode driver circuit 60.

Although different laser diode drivers can be used in accordance withthe present invention, in one aspect of the present invention, the lowpower laser diode driver is illustrated in FIG. 6, and can be used withthe tunable receiver of the present invention. This low power laserdriver circuit 60 can be used for driving the optical preamplifier andreceiver assembly shown in FIG. 4.

A five volt supply voltage input is standard with many electroniccircuits. The laser driver circuit 60 includes an injection laser diode62 that is, in one aspect of the present invention, a high quantumefficiency injection laser diode (HQEILD). A current source control loopcircuit 64 is connected to the injection laser diode 62 and establishesa fixed current through the injection laser diode. This current sourcecontrol loop circuit 64 has a voltage switcher circuit chip 66 connectedto the injection laser diode, within the current source control loopcircuit, and is adapted to receive the fixed supply voltage of fivevolts and convert inductively the supply voltage down to a forwardvoltage, to bias the laser injection diode and produce an optical outputhaving minimized power losses.

This voltage switcher circuit chip 66 is monolithically formed as asingle circuit chip, and is used as a high efficiency voltage converteras shown in FIG. 6.

The current source control loop circuit 64 includes the high efficiencycurrent source 70, acting as a low noise current source and the currentcontrol circuit 72. These circuits are all contained within one housing,and in one aspect, on a printed circuit card assembly 74 that includesthe receiver components, including the preamplifier, tunable bandpassfilter circuit and optical-to-electrical conversion circuit.

The schematic circuit diagram shows various power and voltage, as wellas current parameters. In this non-limiting example, at 260 milliwattsand at five volts DC, there is a 35 decibel optical gain, with onechannel as a design goal. There could be a 266 milliwatt DC for eightchannels, and 220 milliwatts DC achieved. The Bragg grating 73 isoperatively connected to the injection laser diode 62, and is operativeby principles known to those skilled in the art. The Bragg grating 73 isconfigured for receiving the optical output and stabilizing the opticalwavelength.

As shown in FIG. 4, an ASE Reduction Stage circuit 80 works inconjunction with an isolator circuit 82 using amplification techniquesknown to those skilled in the art. The tunable filter section 56includes the tunable filter 46, which in one aspect of the presentinvention, is a fiber Fabry Perot tunable filter 84. A 1:99 coupler 88,as a non-limiting example, allows a portion of the optical signal to beconverted by a photodetector to an electrical current, and pass into afeedback control circuit 87, including an analog/digital converter 88, alow power controller 90, which is operative with a controller interface92 and associated electronics, and digital/analog converter 94, forconverting digitally processed control signals back to analog controlsignals and selectively tuning the fiber Fabry Perot tunable filter.This circuitry also allows an optical spectrum to be detected andstored.

The optical communications signal, once tuned to the desired wavelengthand frequency, passes into the receiver section 58 that includes anoptical-to-electrical conversion circuit having the detector, i.e, thePIN photodiode 48, followed by the low noise electrical amplifier 50,which in one aspect of the invention, is a preferred transimpedanceamplifier and amplifies the converted electrical communication signalreceived from PIN photodiode 48. An electronic limiter circuit 96receives the electrical communications signal and works in conjunctionwith a clock and data recovery circuit 52. This circuit allows datarecovery and reshaping of electrical communication signals. A clockrecovery circuit portion of circuit 52 allows recovery of clock signalsand retiming of electrical communication signals by techniques known tothose skilled in the art.

The data is output to the back plane 20 as shown in FIG. 1. In theembodiment shown in FIG. 2, the signal is sent to the processingequipment 32, and tunable transmitter 34, which then passes the signalback onto the main optical communication path 13.

In one non-limiting example of the present invention, the amplifiersection 54 has about 230 mW with commercial off the shelf components(COTS) of about 2.0 watts, followed by the tunable filter section 56operable at about 50 mW and COTS of about 6 W, and the receiver section58 of about 680 mW and COTS of 1.5 W for a 2.5 Gb/S data rate.

The optical sensitivity at 2.5 Gb/S can be less than about −40 dBm at1×10⁻¹⁰ BER (bit error rate) with a total one channel power consumptionof about 960 mW. For a non-limiting example of eight channels, it ispossible to use a fixed λ demultiplexer providing a total powerconsumption of about 5.7 W, corresponding to 710 mW/channel.

Referring now to FIG. 5, there is illustrated the tunable filter thathas been modified to have channel monitoring or optical spectrumanalysis capability in accordance with another aspect of the presentinvention. Between the analog/digital conversion circuit 88 and the lowpower controller 90, an optical channel monitoring circuit 100 isconnected. The circuit 100 can include a spectrum analyzer, power meteror other associated electronic equipment for monitoring the channel.Thus, it is possible to select various wavelengths to monitor theoperation of the particular channel and determine if there are errors indata transmission or other selected aspects. In this aspect of theinvention, the tunable filter can be swept and the optical power storedin a processor memory, controller or other means known to those skilledin the art. This data can be processed and subsequently displayed,providing an optical spectrum analysis of the signal. The system canmonitor averaged power and supervisory communications data. It also canperform an optical spectrum analysis of the signals.

It is also possible to use the optically amplified back-up receiver as atracking filter for systems that use the tunable laser for lasertransmitters that fail. The receiver can track to a new wavelengthlocation where the tunable transmitter has been positioned to accountfor failing or failed laser transmitter.

This application is related to copending patent application entitled,“OPTICALLY AMPLIFIED BACK-UP RECEIVER,” which is filed on the same dateand by the same assignee and inventors, the disclosure which is herebyincorporated by reference.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that themodifications and embodiments are intended to be included within thescope of the dependent claims.

That which is claimed is:
 1. A system for monitoring the performance ofoptical communications channels comprising: an optical splitterpositioned along an optical communications path for receiving awavelength division multiplexed (WDM) optical communications signal onthe optical communications path and splitting a portion of the WDMoptical communications signal into a low power WDM signal onto a back-uppath; a tunable filter that receives the low power WDM optical signaland selects a desired optical wavelength; a controller operativelyconnected to said tunable filter for controlling the selection of adesired wavelength corresponding to an optical communications channel;an optical amplifier for receiving the low power WDM signal aftersplitting and amplifying same, said amplifier including an injectionlaser diode and circuit for converting inductively a supply voltage, toa forward biasing voltage to the injection laser diode, a current sourcecontrol loop circuit connected to said injection laser diode thatestablishes a fixed current through the injection laser diode, and avoltage switcher circuit connected to said injection laser diode andcurrent source control loop circuit; and an optical channel monitoringcircuit operatively connected to said controller for monitoring theoptical communications channels for performance by sweeping the tunablefilter and processing data for optical signal analysis.
 2. A systemaccording to claim 1, wherein said tunable filter is swept and theoptical power is stored, and subsequently displayed, providing anoptical spectrum analysis of the signal.
 3. A system according to claim1, wherein the tunable filter further comprises a Fabry Perot filter. 4.A system according to claim 1, wherein said controller is operativelyconnected to said tunable filter in a controller feedback path forcontrolling the selection of desired wavelengths corresponding to theoptical communications channels.
 5. A system according to claim 4,wherein said optical channel monitoring circuit is operatively connectedto said controller within said controller feedback path.
 6. A systemaccording to claim 5, wherein said controller feedback path furthercomprises analog/digital converter and optical coupler operativelyconnected to said tunable filter and controller and a digital/analogconverter operatively connected to said controller and tunable filter,wherein said monitoring circuit is operatively connected to saidcontroller and said analog/digital converter.
 7. A system for monitoringthe performance of optical communications channels comprising: anoptical splitter positioned along an optical communications path forreceiving a wavelength division multiplexed (WDM) optical communicationssignal on the optical communications path and splitting a portion of theWDM optical communications signal into a low power WDM signal onto aback-up path; a tunable filter that receives the low power WDM opticalsignal and selects a desired optical wavelength; a controlleroperatively connected to said tunable filter for controlling theselection of a desired wavelength corresponding to an opticalcommunications channel; an optical channel monitoring circuitoperatively connected to said controller for monitoring the opticalcommunications channels for performance by sweeping the tunable filterand processing data for optical signal analysis; an optical amplifierfor receiving the low power WDM signal after splitting and amplifyingsame, said amplifier including an injection laser diode and circuit forconverting inductively a supply voltage to a forward biasing voltage forthe injection laser diode, a current source control loop circuitconnected to said injection laser diode that establishes a fixed currentthrough the injection laser diode, and a voltage switcher circuitconnected to said injection laser diode and current source control loopcircuit; and a detector circuit for receiving the optical signal ofdesired wavelength and converting the optical signal into an electricalsignal for amplification and subsequent signal processing.
 8. A systemaccording to claim 7, wherein said detector circuit comprises a PINdiode.
 9. A system according to claim 7, wherein said detector circuitcomprises an Avalanche Photodiode.
 10. A system according to claim 7,wherein said detector circuit comprises an amplifier circuit.
 11. Asystem according to claim 7, wherein said detector circuit comprises alimiter circuit for reshaping the detected optical signal.
 12. A systemaccording to claim 7, wherein said detector circuit comprises a datadecision circuit and clock recovery circuit.
 13. A system according toclaim 7, wherein the tunable filter further comprises a Fabry Perotfilter.
 14. A system according to claim 7, wherein said controller isoperatively connected to said tunable filter in a controller feedbackpath for controlling the selection of desired wavelengths correspondingto the optical communications channels.
 15. A system according to claim14, wherein said monitoring circuit is operatively connected to saidcontroller within said controller feedback path.
 16. A system accordingto claim 15, wherein said controller feedback path further comprisesanalog/digital converter and optical coupler operatively connected tosaid tunable filter and controller and a digital/analog converteroperatively connected to said controller and tunable filter, whereinsaid monitoring circuit is operatively connected to said controller andsaid analog/digital converter.
 17. A method of monitoring theperformance of optical communications channels of a wavelength divisionmultiplexed (WDM) optical communications signal comprising the steps of:splitting off a percentage of the power from the WDM opticalcommunications signal as an optical signal into a back-up path;filtering the optical signal within a tunable filter and controllablyselecting a desired wavelength by a controller and corresponding to arespective optical communications channel; optically amplifying the lowpower WDM signal after splitting by converting inductively a supplyvoltage to a forward biasing voltage and supplying an injection laserdiode, a current source control loop circuit connected to said injectionlaser diode that establishes a fixed current through the injection laserdiode, and a voltage switcher circuit connected to the injection laserdiode and current source control loop circuit; and monitoring theoptical communications channels for performance in an optical channelmonitoring circuit operatively connected to the controller by sweepingthe tunable filter and processing data for optical signal analysis. 18.A method according to claim 17, wherein the percentage of split off fromthe WDM optical communications signal is about five percent.
 19. Amethod according to claim 17, and further comprising the step of tuningthe optical signal within a Fabry Perot tunable filter.
 20. A methodaccording to claim 17, and further comprising the step of controllingthe selecting of desired wavelengths within the tunable filter viacontroller that is operatively connected to the tunable filter in acontroller feedback path.
 21. A method according to claim 17, whereinthe step of monitoring comprises the steps of monitoring within aspectrum analyzer.
 22. A method of monitoring the performance of opticalcommunications channels of a wavelength optical communications signalcomprising the steps of: splitting off a percentage of the power fromthe WDM optical communications signal as an optical signal into aback-up path; filtering the optical signal within a tunable filter andcontrollably selecting a desired wavelength by a controller andcorresponding to a respective optical communications channel; monitoringthe optical communications channels for performance in an opticalchannel monitoring circuit operatively connected to the controller bysweeping the tunable filter and processing data for optical signalanalysis; optically amplifying the low power WDM signal after splittingby converting inductively a supply voltage to a forward biasing voltageand supplying an injection laser diode, a current source control loopcircuit connected to said injection laser diode that establishes a fixedcurrent through the injection laser diode, and a voltage switchercircuit connected to the injection laser diode and current sourcecontrol loop circuit; and detecting and converting the optical signal ofselected wavelength into an electrical communications signal foramplification and subsequent signal processing.
 23. A method accordingto claim 22, wherein the percentage of split off from the WDM opticalcommunications signal is about five percent.
 24. A method according toclaim 22, and further comprising the step of amplifying the detectedoptical signal.
 25. A method according to claim 22, and furthercomprising the step of reshaping the detected optical signal within alimiter circuit.
 26. A method according to claim 22, and furthercomprising the step of tuning the optical signal within a Fabry Perottunable filter.
 27. A method according to claim 22, and furthercomprising the step of controlling the selecting of desired wavelengthswithin the tunable filter via the controller that is operativelyconnected to the tunable filter in a controller feedback path.
 28. Amethod according to claim 22, wherein the step of monitoring comprisesthe steps of monitoring within a spectrum analyzer.
 29. A system formonitoring the performance of optical communications channelscomprising: an optical splitter positioned along an opticalcommunications path for receiving a wavelength division multiplexed(WDM) optical communications signal on the optical communications pathand splitting a portion of the WDM optical communications signal into alow power WDM signal onto a back-up path; a tunable filter that receivesthe low power WDM optical signal and sweeps the optical communicationschannels; a monitoring circuit operatively connected to said tunablefilter for monitoring the optical communications channels forperformance; an optical amplifier for receiving the low power WDM signaland amplifying same after splitting from the optical communicationssignal, and comprising: an injection laser diode; a current sourcecontrol loop circuit connected to said injection laser diode thatestablishes a fixed current through the injection laser diode; and avoltage switcher circuit connected to said injection laser diode andcurrent source control loop circuit.
 30. A system for monitoring theperformance of optical communications channels comprising: an opticalsplitter positioned along an optical communications path for receiving awavelength division multiplexed (WDM) optical communications signal onthe optical communications path and splitting a portion of the WDMoptical communications signal into a low power WDM signal onto a back-uppath; a tunable filter that receives the low power WDM optical signaland sweeps the optical communications channels; a controller operativelyconnected to said tunable filter in a controller feedback path forcontrolling the selection of desired wavelengths corresponding to theoptical communications channels, said controller feedback path furthercomprising analog/digital converter and optical coupler operativelyconnected to said tunable filter and controller and a digital/analogconverter operatively connected to said controller and tunable filter,wherein said monitoring circuit is operatively connected to saidcontroller and said analog/digital converter; and a monitoring circuitoperatively connected to said tunable filter and controller within saidcontroller feedback path for monitoring the optical communicationschannels for performance.
 31. A method of monitoring the performance ofoptical communications channels of a wavelength division multiplexed(WDM) optical communications signal comprising the steps of: splittingoff a percentage of the power from the WDM optical communications signalas an optical signal into a back-up path; sweeping the opticalcommunications channels by filtering the optical signal within a tunablefilter and selecting desired wavelengths corresponding to respectiveoptical communications channels; monitoring the optical communicationschannels for performance; amplifying the optical signal in an amplifiercircuit comprising: an injection laser diode; a current source controlloop circuit connected to said injection laser diode that establishes afixed current through the injection laser diode; and a voltage switchercircuit connected to the injection laser diode and current sourcecontrol loop circuit.